In conventional engine starters, it has been customary to arrange the output shaft, which carries an axially slidable pinion adapted to mesh with a ring gear, and the solenoid device for axially driving the pinion, in a mutually parallel relationship. In such a bi-axial engine starter, the solenoid device extends radially from the electric motor and therefore inevitably makes the radial dimension of the starter substantial, imposing severe restrictions in ensuring a sufficient space for mounting the engine starter. For this reason, various types of coaxial starters have been proposed in which an annular solenoid device is arranged so as to surround the output shaft (see for example Japanese Patent Application Laid-Open Publication No. 8-319926 filed by the same applicant).
In either type of engine starter, the electric motor is surrounded by a yoke which also serves as a cover. Typically, a reduction gear unit is disposed between the electric motor and the pinion, and between the electric motor and the reduction gear unit is interposed an annular separator, while the end of the electric motor facing away from the separator is covered by a bottom member. In the engine starter proposed by the applicant of this invention, a disk-shaped bottom plate which can be formed by means of press working is used as the bottom member in order to reduce the manufacturing cost thereof.
In such an engine starter, one end of the yoke is fitted on axial protrusions formed on the bottom plate in such a manner that the inner surface of the one end portion of the yoke engages the outer surfaces of the protrusions of the bottom plate, while the other end of the yoke is fitted into a cover member (gear cover) for surrounding the pinion and the reduction gear unit, with the outer surface of the other end portion of the yoke engaging the inner surface of the cover member so that the yoke is securely held, and thereby achieving the axial alignment of the cover member, the yoke and the bottom plate. In such an arrangement, therefore, both outer and inner surfaces of the yoke must be formed with high precision by press working or finished by machining with high precision after the press working. This may complicate the manufacturing process and therefore lead to a higher manufacturing cost.